This invention relates to antireflective (AR) coatings for vitreous substrates and, more particularly, to such antireflective coatings which are graded in porosity in order to be effective over a wide band of radiations, a coating solution for applying the antireflective coating, and a method for making the antireflective coating.
The subject of gradient-index optics apparently dates back to the 1850's and is summarized in article entitled "Gradient-Index Optics: A Review" by Moore, Applied Optics, Volume 19, No. 7, April 1980, pgs. 1035-1038. In an article entitled "Single-Layer, Gradient Refractive Index Antireflection Films Effective From 0.35 Micron To 2.5 Micron" by Minot, Journal Optical Society of America, Vol. 66, No. 6, June 1976, pgs. 515-519 is reported so-called phase separable glasses wherein alkali borosilicate glass is heated to a relatively high temperature to cause it to separate into phases, with the surface portion thereof then etched to selectively dissolve portions of the phase-separated material to leave a residual skeletal layer made up largely of silica. Various patents describe this development, such as U.S. Pat. No. 4,019,884 dated Apr. 26, 1977 to Elmer and Walters and U.S. Pat. No. 4,086,074 dated Apr. 25, 1978 to Minot et al. A further improvement for such a coating is set forth in U.S. Pat. No. 4,080,188 dated Mar. 21, 1978 wherein the etched material is thereafter heated again to cause the substrate to become homogeneous.
The theoretical and mathematical analyses of such thin films are outlined in article entitled "Reflectance Of An Inhomogeneous Thin Film" by Monaco, Journal of the Optical Society of America, Vol. 51, No. 3, March 1961, pgs. 280-282. A further analysis of these porous antireflective films is set forth in article entitled "Development Of Porous Antireflective Films On Borosilicate Glasses" by Iqbal et al., Journal of the Am. Ceramic Soc., Vol. 66, No. 4, pgs. 302-207 (1983). Particular attention is directed to FIG. 1 of the Iqbal reference which illustrates in schematic form the graded refractive index which occurs between the air-layer interface and between the layer-glass interface which results from the graded porosity. Apparently the alkali borosilicate glasses of Minot et al. cannot be used with high power lasers. This is discussed in article entitled "Graded-Index Antireflection Surfaces For High-Power Laser Applications" by Lowdermilk et al, Applied Physics Letter 36 (11), June 1980, pages 891-893.
The deposition of porous oxides from metal-organic derived precursors as antireflective coatings is described in article entitled "Investigations Of Porous Oxides As An Antireflective Coating For Glass Surfaces" by Yoldas, Applied Optics, Vol. 19, No. 9, May 1980, pgs. 1425-1429. These coatings are not graded. The preparation of mixed titania-silica antireflecting coatings from mixed alkoxides is disclosed in article entitled "Antireflecting Coatings Applied From Metal-Organic Derived Liquid Precursors" by Yoldas et al., Applied Optics, Vol. 18, No. 18, September 1979, pages 3133-3138.
The controlled hydrolysis and polymerization of silicon alkoxides by varying the parameters of preparation is disclosed in article entitled "Introduction And Effect Of Structural Variations In Inorganic Polymers And Glass Networks" by Yoldas, Journal of Non-Crystalline Solids, 51 (1982), pgs. 105-121.